The present invention relates to electrolytic cells comprising polymeric film composition electrodes and separator membranes and to a manner of using such cells to provide highly efficient and economical batteries. In particular, the invention relates to unitary rechargeable lithium battery cells comprising an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell.
The invention is particularly useful for making such cells in which the ion source electrode is a material, such as a transition metal oxide, capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility. More specifically, the present invention relates to a construction and arrangement of such battery cell elements which significantly reduces the internal resistance of the resulting battery while improving substantially the level of power capacity available in such a battery.
Prior rechargeable lithium ion battery cells, such as described in the incorporated disclosures, have generally been constructed by means of the lamination of electrode and separator/electrolyte cell elements which are individually prepared, by coating, extrusion, or otherwise, from compositions comprising polymeric materials, e.g., a plasticized polyvinylidene fluoride (PVdF) copolymer. For example, in the construction of a lithium-ion battery, a current collector layer of aluminum foil or grid was overlaid with a positive electrode film or membrane separately prepared as a coated layer of a dispersion of intercalation electrode composition, e.g., a LiMn.sub.2 O.sub.4 powder in a copolymer matrix solution, which was dried to form the membrane. A separator/electrolyte membrane formed as a dried coating of a composition comprising a solution of the copolymer and a compatible plasticizer was then overlaid upon the positive electrode film. A negative electrode membrane formed as a dried coating of a powdered carbon dispersion in a copolymer matrix solution was similarly overlaid upon the separator membrane layer, and a copper collector foil or grid was laid upon the negative electrode layer to complete a cell assembly. This assembly was then heated under pressure to effect heat-fused bonding between the plasticized copolymer matrix components and to the collector grids to thereby achieve lamination of the cell elements into a unitary flexible battery cell structure.
The resulting laminated battery structure, which comprised a significant measure of homogeneously distributed organic plasticizer, particularly in the separator membrane stratum, was devoid of hygroscopic electrolyte salt and, as a result, could be stored at ambient conditions, either before or after being shaped or further processed, without concern for electrolyte deterioration due to reaction with atmospheric moisture. When it was desired to activate a battery in the final stage of manufacture, the laminate cell structure was immersed in or otherwise contacted with an electrolyte salt solution which imbibed into the copolymer matrix to provide substantially the same ionic conductivity enhancement as achieved by a preformed hybrid separator/electrolyte film containing such an electrolyte salt solution.
In order to facilitate the absorption of electrolyte solution during activation, it is generally preferred that a substantial portion of the plasticizer be previously removed from the copolymer matrix. This may readily be accomplished at any time following the laminating operation by immersion of the cell laminate in a copolymer-inert, low-boiling solvent, such as diethyl ether or hexane, which selectively extracts the plasticizer without significantly affecting the copolymer matrix of the cell element strata. The extracting solvent may then simply be evaporated to yield a dry, inactive battery cell which will readily absorb an effective amount of electrolyte solution that essentially replaces the extracted plasticizer.
As with any electrolytic cell, a lithium-ion cell generally prepared in the foregoing manner exhibits a characteristic internal electrical resistance which is ordinarily a function of the various composition materials and the amounts, i.e., the mass or thickness, of each employed in the cell. We were particularly surprised, therefore, in discovering that the internal resistance and performance of such cells having elements of substantially similar composition and mass could be significantly varied by means of the physical structure of the cell and disposition of the component materials within the cell. Arrangement of the cell components in accordance with the present invention has enabled a notable reduction in the internal resistance of the battery cells without compromising specific capacity and stability.